Back in the 19th century, Albert Howard, considered one of the fathers of composting, was a big fan of adding lime to compost.
In recent decades, though, the practice has been discouraged by many composters who see it as unnecessary.
After all, lime is alkaline. While compost goes through some major pH swings, it usually ends up with a fairly neutral or slightly alkaline pH.
One study did find that adding lime could have some major benefits for composting – potentially increasing composting speed and reducing the amount of heavy metals.
Lime Can Increase Compost Speed and Maturity Finds Study
One study by Chinese researchers tested adding lime along with a fertilizer (calcium magnesium phosphate*) could have some major benefits for composting.
The researchers tested adding lime varying in strengths from 0.5% to 1.5%, alongside 10% calcium magnesium phosphate, to a compost made from farm and food waste.
The addition led to some pretty interesting results! The researchers found that there was an increase in beneficial bacteria in the compost, which they believe in turn led to faster compost maturity and higher germination rates.
What’s more, the combination of lime and fertiliser meant more nitrogen was retained in the compost – a process the researchers put down to an increase in the pH level.
This pH level meant that there were decreases in a harmful gas, nitrous oxide, which is one of the ways nitrogen escapes from compost. That meant there were two benefits – a more nutritious compost and reduced greenhouse gases.
It wasn’t the only reduction in gases either.
The researchers noted that compost with 0.5% and 1.0% lime showed large reductions in methane and ammonia emissions – two gases which are harmful to the environment. It did this by limiting the growth of bacteria that produce these gases.
Going up to 1.5% lime, though, did cause problems with the amount of hydrogen sulfide – a harmful gas that smells like rotten eggs – starting to increase.
It’s not the only study to find benefits of adding lime. Singh and Kalamdhad found that adding up to 2% of lime to compost reduced the leaching and availability of heavy metals in compost.
So, should you add lime to your compost?
On balance, though, it does sound like Albert Howard may have been on to something with the addition of lime, so should we add it?
Probably not yet, at least for home composters.
Firstly, composting works perfectly well without lime. For the home composter, there’s no need to add expense and complexity to an otherwise simple process by adding lime.
Secondly, the research using lime has been limited. Even then, the benefits of the first study came about when the lime was combined with another fertilizer – and at 10%, you’d need quite a lot of it if you have a large compost heap.
Finally, if you do add lime, and get the amount wrong, you could cause problems. The first study here found that adding more than 1.5% lime created bad odors. Too much lime can also impact the microbes that work their magic in composting, and reduce the quality of the finished material.
If you’re a home composter, it’s probably best to focus on the basics of effective composting, or at least limit yourself to more proven compost hacks covered on this website.
Still, it’s a reminder to keep an open mind when it comes to composting practices – and be ready to learn from research! In the meantime, if you want to improve your composting, check out these 17 ways to improve compost speed!
* Calcium magnesium phosphate is a slow-release fertilizer that contains calcium, magnesium, and phosphorus.
Sources
Singh, J., & Kalamdhad, A. S. (2013). Effects of lime on bioavailability and leachability of heavy metals during agitated pile composting of water hyacinth. Bioresource Technology, 138, 148–155. https://doi.org/10.1016/j.biortech.2013.03.151
Zhang, L., Shi, T., Xu, Z., Bao, Z., Li, J., Li, G., & Yuan, J. (2023). Effect of lime and calcium magnesium phosphate on gaseous emissions, maturity, and bacterial dynamics during food waste composting. Environmental Technology & Innovation, 103306. https://doi.org/10.1016/j.eti.2023.103306
Read more
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FAQs
Do you need to add lime to compost?
No, a well designed compost bin or heap works perfectly well without adding any lime. It’s true that early advice often suggested adding lime, but modern advice has swung against this, and there’s not enough evidence from research to suggest adding lime can add major benefits.
How much lime should you add to compost?
Limited research suggests adding up to 1.5% to compost may increase compost speed and reduce heavy metal availability. Adding more may lead to bad odors and reduce the quality of the compost. However, do note we don’t recommend adding lime at present as there is not enough evidence to demonstrate proven benefits.
Is lime necessary for composting food waste?
No, food and garden waste can decompose effectively without lime. Proper aeration, moisture control and balancing greens (high nitrogen materials) and browns (high carbon materials) are sufficient for composting food waste at home.
Can you use lime in a composting toilet?
Lime is sometimes used in composting toilets to reduce odors and regulate pH, but this is a different application from traditional composting. Specific types of lime, such as hydrated lime, are better used for this purpose.
What is the difference between lime and other compost accelerators?
Lime primarily works by adjusting pH, whereas compost accelerators often add nutrients or microbes to enhance decomposition. Compost accelerators are likely more effective, and can also be made at home – here are six you can try for free.
Are there risks to using lime in compost?
Risks include over-application, leading to harmful gas emissions, bad odors, reduced compost quality, and disruption of the composting process. Home composters should be cautious and consider proven methods instead.